1. Field of the Invention
The present invention relates to a liquid crystal display device using plastic substrates.
2. Description of the Related Art
Liquid crystal display devices including a liquid crystal panel are generally used as displays of watches, calculators, TV sets, personal computers, word processors, navigation systems, instruments, other information apparatus, and terminals thereof. The liquid crystal panel is mainly composed of glass substrates. There is a reflection type liquid crystal display device that has just been put into practical use, where a reflecting layer and a color filter layer are formed on the inner surface of a glass substrate on the back of the liquid crystal panel, while a single polarizing plate is formed on the outer surface of the other glass substrate on the front.
In recent years, for reducing the thickness and weight of the device, it has been proposed to use plastic substrates in place of the glass substrates to construct a reflection type liquid crystal display device as described above. Such a liquid crystal display device using plastic substrates also includes a single polarizing plate on the front of the liquid crystal panel.
The liquid crystal display device using plastic substrates described above has the following problem. The polarizing plate formed on the front of the liquid crystal panel expands and contracts with changes in temperature and humidity. This causes deformation such as warping and distortion in the liquid crystal device, and the entire device is deformed into a propeller-like shape or an irregular shape. Thus, the performance of the liquid crystal display device as a display is obstructed, making it difficult to provide a plastic display having long-term stability. The reason for expansion and contraction of the polarizing plate is that a polarizing film, which is an oriented film, expands and contracts in the orientation axis direction with changes in temperature and humidity.
In order to solve the above problem, Japanese Unexamined Patent Publication JP-A 8-54620 (1996) proposes a method where a plastic substrate having high optical transparency is integrally formed on a surface of a polarizing plate. The polarizing plate is provided on the front or back of a liquid crystal panel having a pair of plastic substrates. Japanese Unexamined Patent Publication JP-A 11-305040 (1999) proposes a method where an ultraviolet curing resin layer is formed on a polarizing plate.
However, the reflection type liquid crystal display device described above, to which the methods disclosed in JP-A 8-54620 and JP-A 11-305040 are applied, is of an asymmetrical structure having a polarizing plate only on the front. With this structure, warping of the polarizing plate is not sufficiently suppressed. As a result, there arises a problem of generating warping and distortion in the liquid crystal display device.
An object of the invention is to provide a liquid crystal display device using plastic substrates in which deformation such as warpage and distortion does not easily appears.
The invention provides a liquid crystal display device comprising a light transmitting first plastic substrate, a second plastic substrate including a reflecting layer, a liquid crystal layer interposed between the first and second plastic substrates, and a polarizing plate formed on an outer surface of the first plastic substrate, the liquid crystal display device further comprising a third plastic substrate formed on an outer surface of the second plastic substrate.
In a structure having a polarizing plate only on one side of the liquid crystal display device, specifically, on the outer surface of the first plastic substrate, the liquid crystal display device may possibly be deformed as the polarizing plate expands and contracts with changes in temperature and humidity. According to the invention, however, the third plastic substrate, which expands and contracts with changes in temperature and humidity, is formed on the side of the liquid crystal display device opposite to the polarizing plate, that is, on the outer surface of the second plastic substrate. Therefore, when the polarizing plate expands and contracts with changes in temperature and humidity, the third plastic substrate on the second substrate also expands and contracts in the same manner. This suppresses deformation of the liquid crystal display device.
Accordingly, the liquid crystal display device of the invention is prevented from deformation such as warping into a propeller-like shape or distortion into an irregular shape, and thus can be assembled reliably for long-term use. As the third plastic substrate that expands and contracts with changes in temperature and humidity, an oriented plastic substrate made of polycarbonate, for example, or a non-oriented plastic substrate made of an epoxy resin, may be used.
As described above, according to the invention, the third plastic substrate that expands and contracts with changes in temperature and humidity is provided on the outer surface of the plastic substrate provided with the reflecting layer. When the polarizing plate expands and contracts with changes in temperature and humidity, the third plastic substrate also expands and contracts. This suppresses deformation of the liquid crystal display device. Therefore, the liquid crystal display device of the invention has a structure that can remarkably reduce deformation such as warping and distortion generated with changes in temperature and humidity, and can maintain stable quality for a long period of time.
In the invention it is preferable that the third plastic substrate includes an oriented film.
According to the invention, the third plastic substrate formed on the second plastic substrate includes an oriented film. Therefore, the third plastic substrate expands and contracts in the orientation axis direction with changes in temperature and humidity, thereby suppressing deformation of the liquid crystal display device. Examples of the plastic substrate comprising an oriented film include: a plastic substrate made of polycarbonate (PC), for example, where a single film can be oriented; and a substrate including layered oriented films such as oriented polyester (e.g., polyethylene terephtalate (PET)) films.
According to the invention, since the third plastic substrate formed on the outer surface of the second plastic substrate includes an oriented film, the third plastic substrate easily expands and contracts with changes in temperature and humidity. This suppresses deformation of the liquid crystal display device.
In the invention it is preferable that the third plastic substrate including an oriented film is a polarizing plate including a polarizing film.
According to the invention, the third plastic substrate is a polarizing plate including a polarizing film. Therefore, the polarizing plate expands and contracts with changes in temperature and humidity as the polarizing plate formed on the first plastic substrate does. This suppresses deformation of the liquid crystal display device.
Furthermore, in the invention it is preferable that an orientation axis direction of the oriented film is parallel to an orientation axis direction of the polarizing plate formed on the outer surface of the first plastic substrate.
Furthermore, in the invention it is preferable that an orientation axis direction of the oriented film is parallel to an orientation axis direction of a polarizing film of the polarizing plate formed on the outer surface of the first plastic substrate.
According to the invention, the orientation axis direction of the third plastic substrate including the oriented film is parallel to the orientation axis direction of the polarizing plate formed on the first plastic substrate. Therefore, the third plastic substrate expands and contracts in the same directions as the polarizing plate, thereby suppressing deformation of the liquid crystal display device.
According to the invention, the orientation axis direction of the third plastic substrate as a polarizing plate is parallel to the orientation axis direction of the polarizing plate formed on the first plastic substrate. Therefore, the third plastic substrate expands and contracts in the same directions as the polarizing plate formed on the first plastic substrate with changes in temperature and humidity. This suppresses deformation of the liquid crystal display device.